Rosobolus, Havlicek, 1982

Hansen, Jesper & Holmer, Lars E., 2011, Taxonomy and biostratigraphy of Ordovician brachiopods from northeastern Ny Friesland, Spitsbergen 3076, Zootaxa 3076 (1), pp. 1-122 : 31-33

Rosobolus ? elongatus sp. nov.

Pl. 5, Figs. 11–20; Table 5

Derivation of name. Latin ‘ elongate ’, elongate; refers to the more elongate outline compared to other species within the genus.

Holotype. Pl. 5, Figs. 14–15; TSGF16958 , dorsal valve; 97 m above base of Profilbekken Member, Valhallfonna Formation, sample JH-136; Profilstranda at Basissletta, Ny Friesland, Spitsbergen.

Material. 10 dorsal, 12 ventral and 58 undetermined valves and one fragment from the sample with the trilobite A84270 and from samples F3475, F3478, F3518, F3675.5 and JH-136. The paratypes are TSGF16955–16957 and TSGF16959 .

Diagnosis. Rosobolus with an acuminate, egg-shaped outline; dorsal pseudointerarea low, divided by a deep median groove; visceral area weakly impressed; dorsal vascula lateralia submarginal; micro-pitting 2 µm in diameter or slightly less.

Description. Shell slightly to moderately convex with acuminate, egg-shaped outline. Larval shell acuminate to subcircular. Shell widest at 49–62% of valve length; L/W ratio 1.15–1.43. Largest measured specimen 1.8 mm long and 1.4 mm wide. Larval shell ornamentation consisting of dense, shallow pits about 1 µm in diameter. Postlarval shell with macro-ornamentation of fine growth lines. Micro-ornamentation of rather uniform and dense, shallow, transversely rhomboidal pits up to 2 µm in diameter. Valve floor generally not pitted.

Dorsal valve highest at 29–38% of valve length. Pseudointerarea orthocline, smooth and merging with valve floor. Pseudointerarea about 26% as long and 67% as wide as valve. Flexure lines absent. Propareas declining toward valve floor. Median groove deep, with nearly straight sides and concave anterior margin. Median groove slightly raised above valve floor, 6–9% as long and 21–37% as wide as valve. Visceral area weakly impressed. Impressions of submarginal vascula lateralia nearly straight, a little over 25–35% of valve width from lateral valve margin at central one-third of valve length. Vascula media subparallel along median ridge, weakly divergent afterward. Median ridge absent or very weakly developed in central one-third of valve. Anterior lateral muscle scars subcircular, located at 57% of valve length.

Ventral valve highest at 43% of valve length. Pseudointerarea low, orthocline, not or only partly merging with valve floor. Flexure lines strong, nearly straight. Inner propareas small and triangular with strong growth lines. Outer propareas large, higher than inner propareas, smooth or with fine growth lines. Pseudointerarea 53–62% as wide and 17–24% as long as valve. Pedicle groove shallow with straight sides diverging at about 40–45º and convex to concave anterior margin weakly raised above valve floor. Pedicle groove 10% as long as valve. Visceral area weakly defined. Central muscle scars large, weak, transversely rectangular and nearly normal on the midline. Pedicle nerve impressions weak, subparallel, reaching 43% of valve length.

Remarks. This species shows the strongest similarity to other Rosobolus . However, it is distinguished by two important characters. The dorsal pseudointerarea is high, shelf-like and not divided by a median groove in Rosobolus . Another diagnostic feature of Rosobolus that differs in the Spitsbergen species is the marginal dorsal vascula lateralia. The distinctive micro-ornamentation is also known from several genera within the family Elkaniidae Walcott & Schuchert , but these genera are distinguished by characters such as strongly biconvex, broader, thickwalled shells with rugae and strongly defined visceral areas. This species will probably have to be placed in a new genus.

Occurrence. 80–85, 90–92, and 97–98 m above base of Olenidsletta Member, Valhallfonna Formation, Basissletta in northeastern Ny Friesland, Spitsbergen.

Plate 5

Obolid sp. 5

Valhallfonna Formation, Profilbekken Member, 67 m above base.

1. TSGF16879 , valve exterior. Coll. J. Hansen, 24.07.2008, sample JH-95 .

2–3. TSGF16878 , valve exterior and detail of ornamentation. Coll. J. Hansen, 24.07.2008, sample JH-95 .

4. TSGF16877 , valve interior. Coll. J. Hansen, 24.07.2008, sample JH-95 .

Zhanatellidae indet.

Valhallfonna Formation, Olenidsletta Member, 113 m above base. Coll. J. Hansen, 31.07.2008, sample JH-157.

5–8. TSGF16787, ventral exterior, detail of postlarval micro-ornamentation close to larval shell, detail of postlarval microornamentation along midline in anterior half, detail of postlarval micro-ornamentation on lateral part of shell.

9–10. TSGF16788, ventral valve interior, detail of pseudointerarea.

Rosobolus ? elongatus sp. nov.

Valhallfonna Formation, Olenidsletta Member, 97 m above base. Coll. J. Hansen, 28.07.2008, sample JH-136.

11–13. TSGF16957 , paratype, dorsal valve exterior, detail of larval micro-ornamentation, and detail of postlarval microornamentation .

14–15. TSGF16958 , holotype, dorsal valve interior, oblique ventrolateral view .

16. TSGF16959 , paratype, detail of dorsal pseudointerarea .

17. TSGF16956 , paratype, detail of ventral pseudointerarea .

18–20. TSGF16955 , paratype, oblique ventrolateral and ventral views of dorsal valve interior and detail of pseudointerarea .